The central objective of this project is to elucidate the fundamental factors involved in the function of redox metalloproteins. We want to understand how these proteins facilitate electron transfer and small molecule activation, how they exert kinetic or thermodynamic control, and how they conserve or utilize the energy of the processes that they catalyze. Our approaches emphasize vibrational spectroscopies which are sensitive to structure. In particular, we emphasize time-resolved vibrational probes. The following Specific Aims are proposed. Heme-Copper Oxidases. (i) We want to understand the differences among oxidases from different species. (ii) We want to check whether the ligation mechanisms that we have proposed for CO apply to O2. (iii) We want to test the hypothesis concerning the relationships between the coordination chemistry of the metal centers, and the control of exogenous ligand entry, redox reactivity and thermodynamics, and the coupling of redox energy to proton translocation. NiFe Hydrogenases. The functional active site of NiFe hydrogenases contains intrinsic CN- and CO, perhaps the most-studied vibrational chromophores in chemistry. This presents an opportunity to use vibrational spectroscopies to address structural and functional issues. These include: the oxidation state changes of Ni and Fe; the electronic structures and the stereochemistry of the binuclear site in various forms, and their changes during reactivity; whether dihydrogen or hydride species may be important; and the identity of other redox co-factors. Perchloroethylene (PCE) Dehalogenase. Quite recently a novel enzyme that accomplishes reductive dehalogenation of a wide range of substrates, PCE dehalogenase, has been isolated and characterized. This is a soluble Co- corrinoid protein with Fe-S clusters as redox cofactors. We want to exploit the Raman and infrared signatures of Co-corrins and Fe-S clusters to probe the structures and functional dynamics of the redox sites of this enzyme.